The present invention relates generally to the art of aligning co-rotatable in-line machine shafts which are coupled together for operation by means of a shaft coupling. More particularly, the invention relates to methods for evaluating a multi-configurable alignment fixture or bracket to verify mechanical integrity of the alignment bracket, and to determine the amount of "sag" to be accounted for when subsequently employing readings taken with the alignment bracket.
As is well known, whenever two rotating machine shafts are coupled together, such as the shaft of an electric motor and the shaft of a pump, it is important that the shafts be aligned within predetermined tolerances. Such shafts, when in perfect alignment, have their extended center lines (axes of rotation) coinciding along a straight line. Misalignment can lead to vibration, excessive wear, and ultimate destruction of couplings, bearings, seals, gears and other components.
A number of shaft alignment methods are known, which generally have in common the use of suitable alignment fixtures, also termed alignment brackets. The alignment brackets are employed to measure particular relative displacements (also termed offsets) as the shafts are rotated together through one revolution, taking readings at various angular positions. Traditionally the shafts are stopped at the 0.degree., 90.degree., 180.degree. and 270.degree. angular positions to take readings. However, as disclosed in commonly-assigned related application Ser. No. 07/892,587 filed concurrently herewith by Kenneth R. Piety and Daniel L. Nower and entitled "Shaft Alignment Data Acquisition", the entire disclosure of which is hereby expressly incorporated by reference, readings may be taken at a number of angular positions other than 0.degree., 90.degree., 180.degree. and 270.degree., and in an automated system data may be collected as the shafts are turned smoothly in their normal direction of rotation, such that no counter-rotation is allowed. Each relative displacement is measured between a point referenced to one of the shafts by means of the alignment bracket and a point on the other shaft. Dial indicators are often employed, these dial indicators having a plunger which moves a hand on the face of the dial indicator.
The readings are then used to calculate machine moves which will bring the shafts into alignment. The 0.degree., 90.degree., 180.degree. and 270.degree. angular positions at which readings are conventionally taken lie in geometric planes in which either of the machines, for example the motor, may be moved for purposes of alignment. In particular, the mounting bolts of the machine may be loosened, and the machine may be either moved in a horizontal plane, moved in a vertical plane by placing or removing shims under one or more of the feet of the machine, or both. There are well developed calculation methods and procedures known in the art for determining what machine moves to make to achieve an aligned condition based on measurement of relative displacement (offset) data at the 0.degree., 90.degree., 180.degree. and 270.degree. positions mentioned.
An alignment bracket typically has a base firmly clamped or otherwise affixed to one shaft, and an extension bar or arm extends laterally from the base in a direction generally parallel to the shafts across the coupling over to a reference point adjacent a point on the periphery of the other shaft. A device for measuring displacement, such as a dial indicator, is positioned so as to measure relative displacement in a radial direction (offset) from the reference point to the point on the periphery of the other shaft as the shafts are rotated together while stopping at the 0.degree., 90.degree., 180.degree. and 270.degree. angular positions to take and record readings. The position of the alignment bracket is then reversed so as to be fixedly referenced to the other shaft, establishing a reference point adjacent a point on the periphery of the one shaft, and the procedure is repeated. Alternatively, a pair of alignment brackets may be employed for simultaneous readings.
From the geometry just described, it will be appreciated that the reference point on the alignment bracket attached to the one shaft rotates about the projected centerline (axis of rotation) of the one shaft to define a circle centered on that projected centerline, and vice versa for the other shaft, and that the distance and direction of the distance between the two shaft centerlines as projected can be determined at any transverse plane along the shaft axes. From the thus measured distances and directions of the distances between the two shaft centerlines as projected in two transverse planes, both offset misalignment and angular misalignment components may be calculated.
When readings are taken with the extension bar in an angular position above the shafts or below the shafts, the readings are affected by the amount the brackets sag under the force of gravity due to their weight. To obtain accurate values for misalignment correction, the sag must be subtracted from the measurement before any calculations are made to align the shaft center lines. Bracket sag primarily results from beam deflection of the extension bar, but there can be additional contributions to sag, such as flexibility in the alignment bracket base or in the fastening devices employed to attach the base to the shafts to be aligned. Thus even laser-based alignment brackets which employ beams of light rather than an extension bar are subject to sag to some extent.
It is relevant to note there are a great many specific alignment bracket dimensional configurations which may be achieved, even with a given alignment bracket. Thus, the extension bar may be adjusted to different lengths to suit the particular alignment configuration, and one or more spacer blocks may be employed to space the extension bar in a radial direction away from the shafts in order to provide clearance around a particular coupling. The invention is particularly applicable to such multi-configurable alignment brackets.
In a typical prior art method for measuring sag, the alignment bracket or brackets are set up on a straight segment of pipe deemed to be sufficiently rigid, or on a suitable mandrel, exactly as the bracket is to be later set up during the shaft alignment process. After the brackets are set up, the pipe or mandrel is rotated such that the extension bar of the alignment bracket is above the pipe or mandrel, that is, lying in a vertical plane defined generally by the pipe centerline, which is defined as the 0.degree. shaft angular position. Depending upon the particular type of dial indicator employed, the dial indicator is either zeroed or an absolute reading is taken. Then, the pipe or mandrel is rotated such that the alignment bracket extension bar is below the pipe or mandrel in the 180.degree. angular position, and another reading is taken. Bracket sag is determined from the difference between the two readings.
This prior art method suffers a number of disadvantages. Inherently, error can be induced when the alignment brackets are moved from the straight pipe to the actual shafts being aligned. No matter how careful the user is, bolts and clamps tends to be tightened in a different order, and to a different torque. In addition, the brackets may inadvertently be set up at a shorter or longer distance. All of these factors affect the amount of bracket sag.
Another disadvantage is that it is possible to mount the alignment brackets in a manner such that the data measured is inaccurate, particularly brackets that are multi-configuration brackets. This can be caused by not tightening all clamps properly, by damaged brackets, or a defective dial indicator.